/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution
{
public:
    ListNode* detectCycle(ListNode* head)
    {
        ListNode* slow = head;
        ListNode* fast = head;

        while (fast && fast->next)
        {
            slow = slow->next;
            fast = fast->next->next;

            if (slow == fast)
            {
                ListNode* prev = head;
                while (prev != slow)
                {
                    prev = prev->next;
                    slow = slow->next;
                }
                return prev;
            }
        }
        return nullptr;

    }
};